A Fourier transform spectrometer, commonly designated by the initials “FTS”, is an instrument observing a scene with a relatively low spatial resolution—that is to say size of survey point—and a very fine spectral resolution. It is typically coupled to a so-called “imager” device, working in spectral broadband, whose spatial resolution is finer. The aim of coupling the FTS with an imager is to improve FTS instrument data geo-location.
By way of example, in a meteorological observation satellite, the Fourier transform spectrometer makes it possible to observe the spectrum of the light in the infrared/near infrared region, and its spatial resolution may be of the order of 10 to 20 kilometers. The Fourier transform spectrometer is coupled with an imager of finer resolution, of the order of a kilometer.
Known Fourier transform spectrometers comprise an analogue detector disposed in a pupil plane. More recent Fourier transform spectrometers—called imager FTSs—comprise, as replacement for the analogue detector, a matrix detector placed in an image plane which decomposes a survey point—representing the final resolution of the instrument—into elementary pixels. This decomposition makes it possible notably to perform digital compensation for the field effect or “self-apodization”, or else to employ, in addition to the “FTS mode”, an image mode, that is to say a mode making it possible to image the field of view of a survey point with a fine spatial resolution. This image mode can replace the “imager instrument”. The physical pixels forming such a matrix are typically produced by the technology commonly designated by the initials CMOS corresponding to the conventional terminology “Complementary Metal Oxide Semiconductor”, or CCD technology corresponding to the conventional terminology “Charge-Coupled Device”. In order to optimize the signal-to-noise ratio, the reading of the detector is performed with spatial resolutions that vary according to the mode. Hereafter, the term “macro-pixel” designates the “digital” resolution of the Fourier transform spectrometer in the “FTS mode”, and the term “super-pixel” designates the “digital” resolution in the image mode. A super-pixel can be formed by a physical pixel of the detection matrix, or else by a grouping of physical pixels, for example of 2×2 or 3×3 pixels, according to the spatial resolution requirement needed for the image mode. A macro-pixel defining the spatial resolution of the FTS mode is in practice formed by a plurality of physical pixels of the detection matrix, for example disposed according to an alignment in one or more rows or in one or more columns of physical pixels of the matrix, in the direction of the interferometric fringes so as to preserve a contrast on the modulated signal which is sufficient (>40%) within each macro-pixel.
The data acquisition time, called the “exposure time”, is decomposed, for spectrometers of imager FTS type, into a time which comprises an acquisition phase in FTS mode and an acquisition phase in image mode, the FTS mode acquisition phase covering of the order of 98% of the exposure time.
The FTS data are essentially used to study/monitor the composition of the atmosphere. Now, utilization of the data with this objective is possible only in a clear scene, that is to say for a cloudless observed scene. A scene considered clear typically contains less than 5% of cloud of the surface area of the survey point. Thus, if a survey point contains a proportion of clouds which is greater than a threshold value of the order of 5%, the data can hardly be utilized by the scientific community. Statistically only a low proportion of the acquired data is in a clear scene (<20%).
The requirement therefore exists for a solution which alleviates these drawbacks. The present invention is aimed at remedying the existing limitations by proposing a novel system for dynamic adaptation of spatial resolution for spectrometers.